The present invention relates generally to printers such as ink jet printers, and more particularly to such a printer including a precision sheet transport control method and apparatus for minimizing printed image misregistration and other observable printed image defects.
An ink jet printer of the type frequently referred to as drop-on-demand, has at least one printhead from which droplets of ink are directed towards a recording medium. Within the printhead, the ink is contained in a plurality of channels. Piezoelectric devices or power pulses cause the droplets of ink to be expelled as required, from orifices or nozzles located at the end of the channels. In thermal ink jet printing, the power pulses are usually produced by resistors also known as heaters, each located in a respective one of the channels. The heaters are individually addressable to heat and vaporize the ink in the channels. As a voltage is applied across a selected heater, a vapor bubble grows in that particular channel and ink bulges from the channel nozzle. At that stage, the bubble begins to collapse. The ink within the channel retracts and then separates from the bulging ink thereby forming a droplet moving in a direction away from the channel nozzle and towards the recording medium whereupon hitting the recording medium a spot is formed. The channel is then refilled by capillary action which, in turn, draws ink from a supply container of liquid ink.
The ink jet printhead may be incorporated into either a carriage type printer or a page width type printer. The carriage type printer typically has a relatively small printhead containing the ink channels and nozzles. The printhead is usually sealingly attached to a disposable ink supply cartridge and the combined printhead and cartridge assembly is attached to a carriage which is reciprocated to print one swath of information (equal to the length of a column of nozzles) at a time on a stationary recording medium, such as a sheet of paper or a transparency.
After each such swath is printed, the sheet of paper is transported or advanced forwardly (usually the movement involves stepping or indexing) a distance that is equal to the height of the printed swath or of a portion thereof so that the next printed swath is properly registered in an overlapping or contiguous manner therewith. The procedure is then repeated until an entire page on the sheet is printed.
Conventional sheet transporting or advancing systems in such printers typically have limited precision due to the limited precision of the mechanical components that make up the system. The predictable and inescapable result of such limited precision is image misregistration. It has been found that when printed material includes misregistration defects of even one-half pixel, such defects will be observable. This is becoming more and more of a problem as the printing industry pushes for lower and lower cost printers with finer and finer levels of pixel resolution, (which has now reached and is exceeding 600 dpi). The reason a one-half pixel misregistration defect is observable is because at 600 dpi a one-half pixel error is equal to about 21 microns.